To start, go to your client area which can be found here.
Click the box that is labeled "SERVICES."
Click on the service that you would like to upgrade or downgrade.
Your products page will look the same but may have different products depending on what you have purchased.
Upgrades are available for free Minecraft services with migration included for free.
Click "Upgrade/Downgrade" which can be found at the left of your screen.
You will now see a list of plans that are available for you to upgrade or downgrade to.
Choose the plan that you want to migrate to and continue with checkout.